1. Field of the Invention
The present invention is directed to a medical implant of the type which generates an extracorporeally detectable signal in response to an interrogation signal, as well as to an interrogation signal generator for generating such an interrogation signal.
2. Description of the Prior Art
In a normal follow-up for a pacemaker patient many different working parameters of the pacemaker are tested, e.g. battery status, stimulation threshold, electrode lead impedance and others. The follow-up is made in a hospital at least once per year and the physician uses a bi-directional communication programmer that communicates with the pacemaker by radio waves. The requested information is received by the programmer and analyzed by the physician, which is a difficult and time-consuming job. The programmer is quite expensive and may not always be a part of the ordinary equipment for every small-sized hospital. Battery tests can also be performed by putting a magnet over the pacemaker that changes the stimulation rate in dependence of the battery status. This rate can be seen on an ECG machine or by listening for or feeling the pulse. Devices used for performing battery tests are for instance disclosed in U.S. Pat. No. 54,390,020 and U.S. Pat. No. 4,416,282.
In U.S. Pat. No. 4,390,020 a programmable pacemaker is disclosed capable of operating in several stimulating modes, having battery powered stimulating means and stimulating mode selector means. Sensing and evaluating means, which can be activated externally by means of a magnet, for example at a medical examination, monitor the terminal voltage of the battery and cause the pacemaker via the stimulating selector means to change operation from a first stimulating mode with a programmed stimulation rate to a mode with a fixed stimulation rate, when the terminal voltage decreases below a first threshold value and operate in a second predetermined stimulating mode at a fixed stimulation rate when the terminal voltage decreases below the first as well as a lower second threshold value.
U.S. Pat. No. 4,416,282 discloses a similar pacemaker which also includes sensing and evaluating means for monitoring of the battery capacity with regard to two battery depletion levels. The stimulation rate automatically decreases with the decreasing of the battery capacity below the depletion levels.
In U.S. Pat. No. 4,488,555 a battery condition warning system for a medical implant is known. The warning system generates an audible alarm to warn the patient of an impending battery failure.
In U.S. Pat. No. 4,614,192 an implantable cardiac defibrillator is disclosed, providing, upon magnet-type interrogation, an audible indication to verify the status of the implanted device. To enable the defibrillator to deliver a defibrillating pulse a control circuit must be placed in an active state. To place it in an active state a ring magnet is used to toggle a status flip-flop that emits an enabling signal to the control circuit. At the same time, an audio oscillator is energized by the output signal from the status flip-flop and from a rate circuit, enabling the audio oscillator to emit sounds synchronous with the heart beat if a bipolar electrode is properly positioned within the heart and to emit a continuous tone if the defibrillator is inactive and properly positioned. Absence or presence of an audible tone indicates whether the probe is properly lodged about the right ventricle. The audible indication in the defibrillator disclosed in U.S. Pat. No. 4,614,192 is generated in response of a magnet interrogation and reflects the status of the implant at the time the interrogation is made, i.e. the measurement procedure is performed at that time.
One drawback for many of these known solutions is that some kind of more or less complicated technical equipment is needed, e.g. a programmer or an ECG-machine, not always available in smaller clinics.
Another drawback is that before the implant can respond to an interrogation from an external device, often time-consuming tests has to be performed by the implant.
An object of the Invention is to provide a medical implant capable of immediately generating an response signal that is easy to recognize outside the body by e.g. a stethoscope, and that reflects the present status of at least one predetermined working parameter of the implant. Another object is for the response signal indicates if the working parameter has a satisfactory value or not.
One further object of the invention is to provide an interrogation signal-generating device adapted to work with a medical implant which achieves the above objects.
The above objects are achieved in a medical implant in accordance with the principles of the present invention which contains a predetermined number of working parameter status registers, each containing updated data representing a first state or a second state of a working parameter, the first state indicating that the working parameter has a satisfactory value and the second state indicating that the working parameter has a non-satisfactory value and wherein, in response to detection of an interrogation signal inquiring about a status of a particular working parameter, the medical implant generates a response signal which only identifies the content of the status register for the working parameter which was the subject of the interrogation signal, and the response signal is extracorporeally detectable.
Since the response signal indicates only whether the inquired-about working parameter has a satisfactory value or a non-satisfactory value, it can be very simply extracorporeally detected, such as by a stethoscope or tactilely, depending on the type of response signal which is generated. Since the response signal has only one of two possible states, there is no need to analyze the signal beyond identifying its state.
The above objects are also achieved in an interrogation signal generator, which operates with a medical implant as described above, which is adapted only for one-way signaling to the medical implant. The interrogation signal generator contains only a signal generator, an energy source, a control unit and a selection unit which is used to activate the signal generator to generate an appropriate interrogation signal regarding a particular working parameter.